Auto merge of #84507 - crlf0710:codegen_nonlocal_main_wrapper, r=nagisa

Add primary marker on codegen unit and generate main wrapper on primary codegen.

This is the codegen part of changes extracted from #84062.

This add a marker called `primary` on each codegen units, where exactly one codegen unit will be `primary = true` at a time. This specific codegen unit will take charge of generating `main` wrapper when `main` is imported from a foreign crate after the implementation of RFC 1260.

cc #28937

I'm not sure who should i ask for review for codegen changes, so feel free to reassign.
r? `@nagisa`

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